text:
What is the difference between “affirm” and “acknowledged”?
Recently, I've watched Air Force One and I heard ATC and the pilots say "Acknowledged." instead of "affirm" or "affirmative" as I've learned..
Is it common to say both? Or might "Acknowledged." just throw the controller / pilot off?

"Acknowledged" means I heard you & understood what you said & I take responsibility for the information you just gave me.
"Affirm" and its opposite, "negative," are answers to a yes/no question.
They aren't really interchangeable, although Hollywood script writers are notorious for getting details like that wrong.
An example of correct usage would be, "Callsign 123, is your ride smooth at FL350?" "Callsign 123, affirm." (Answers the question, yes the ride here is smooth.") Then, "Callsign 123, we've had reports of light turbulence at your 12 o'clock and 100 miles." "Callsign 123, acknowledged." (Or, "roger" is more common, but same meaning... either way, I've received & understood the turbulence report.)
To reply "roger" or "acknowledged" to the question "is your ride smooth" is incorrect, because all you'd be saying is, I heard your question. There isn't any "yes" or "no" component to either of those replies. Likewise, to reply "affirm" to the "turbulence ahead" call is wrong because there was no question being asked there. The controller just needs to know that you heard the information so that it doesn't have to be repeated.
Interestingly, the FAA Pilot-Controller Glossary doesn't list "acknowledged" at all, but lists "acknowledge" as a directive: tell me that you heard me.
It also lists "affirmative" and not "affirm" -- that may be more used elsewhere. Also, with the quality of modern radios, the risk of the initial syllable(s) getting lost & the receiver left puzzling over "...ative" has probably diminished over time.

I've never heard of "Acknowledged" being standard phraseology. IMO "Roger" would be the correct word to use in the examples you mention. Do you have a source for "Acknowledged" being correct phraseology?

@expeditedescent - no, I was responding to the OP. As I noted, "roger" is what I find to be more common, although GdD seems to think it is rare. The question was asking about the difference in meaning, so that was the main point I was trying to address, rather than FAA vs UK vs whatever else is out there that I'm not familiar with.

In the US at least, "roger" is the correct term from the pilot/controller glossary, but I do hear "acknowledged" from time to time in casual/colloquial use.

The UK's CAP413 lists AFFIRM specifically, but similarly, lists ACKNOWLEDGE only as a controller request, and not a response

Movies frequently get pilot language wrong, and this is no exception.

Acknowledge is a command, not a response. ATC would instruct a pilot to acknowledge a transmission, and that is then acknowledged
Affirmative is not the correct phraseology, pilots say Affirm only for yes. Negative means no, so affirmative is not said because it could be confused with negative
Roger is another one movies and books often get wrong. Roger just means 'I have received your transmission'. It doesn't mean yes or no, it doesn't mean you have understood the message either. It is also rarely used

"Roger" dates way back to the very early days of radio when there was only Morse code sent by spark-gap transmitters. A station that received a complete message and didn't require any information to be repeated sent "R" back in the reply. When radiotelephone came along, "roger" was used, being the phonetic word for the letter R. So roger means that the complete message was received and was intelligible. If the message was received and was intelligible, I think most people would think it's reasonable to expect that the recipient also understood a plain-language (not in a secret code) message.

So if this were happening today we'd be saying Romeo every time we got told information. Would make a lot of old war movies seem a lot different.

+1   - @rclocher3 It is reasonable to expect that, but CAP413 explicitly states that 'roger' means 'I have received all your transmission' (page 11) https://publicapps.caa.co.uk/docs/33/CAP413%20MAY16.2.pdf

Movies frequently get domain language wrong, the problem isn't specific to piloting.

@GdD Are you saying that, "Affirmative," is incorrect phraseology? If so you're wrong.

In the UK, the phraseology bible is CAP 413 and that does not list the term "Acknowledged" as standard phraseology at all.
The phrase "ACKNOWLEDGE" can be used by the asker to:

Let me know that you have received and understood this message.

And is responded usually with:

Acknowledgements of information should be signified by the use of the
receiving stations’ callsign or Roger callsign, and not by messages such
as: ‘callsign-copy the weather’ or ‘callsign-copy the traffic’

The phrase AFIRM means

YES

One example in CAP413 has of the use of ACKNOWLEDGE is:

BIGJET 347, I am instructed by Her Majesty’s Government that you are to hold at KTN at FL270. Acknowledge

(How very British!)
Another is:

BIGJET 347, I am informed that there may be damage to the port wing tip of your aircraft. It appears that your planned flight is liable to endanger life. Acknowledge

Which does indeed feel like it needs the pilot to positively acknowledge that they have received the message. CAP413 is not explicit on what the response should be but I suspect it would simply be along the lines of

BIGJET 347, Roger, request taxi to stand.

Good question.
The appropriate term in aviation for "Acknowledged" is the term "Roger." The only time I hear the word "Acknowledged" is on Star Trek and other tv shows or movies. As mentioned above, "Acknowledged" is not a Pilot-Controller term, although a controller might ask you to "acknowledge" something he/she has said or asked, in which case you will respond with either the answer, or say Affirmative or Negative, or in some cases, just "Roger" (plus your tailnumber).
"Roger" simply means you have received the transmission. In other words, "Roger" is an acknowledgement of a transmission.
"Wilco" is rarely used, but it means "you have received the message and will comply", some old pilots combine the two in the redundant "Roger wilco."
"Affirm" is a non-standard, but widely used, abbrevication for "Affirmative." Which simply means yes. Usually pronounced "A-firm." It is the opposite of "Negative." Personally, I hate it when people use "Affirm" on the radio, but it's widely understood and controllers generally accept it in place of "Affirmative."
Don't use "Acknowledged" in place of Affirmative or Roger in aviation communication. Better yet, don't use "Acknowledged" at all in aviation. If you mean "yes" say "Affirmative." If you want to say you've received or acknowledged a transmission say "Roger."
Likewise, don't use "Copy" or "10-4" in place of "Roger."

At least in the UK, and I suspect the whole of Europe, Afirm is not an abbreviation of Afirmative, it's the actual correct terminology for "Yes".

@Jamiec that is so interesting. Thank you for this comment. Maybe that is where it has come from. International pilots from EU flying into the US, and American pilots picking up on it. That makes it a little less annoying for me. :)

If you have a poor reception "Afirmative" and "Negative" will sound remarcably similar for two words of opposite meaning. I always supposed that's why EASA introduced "Afirm" as standard phraseology

@Radu094 I see your point because they have similar endings, but I was thinking the opposite. Affirmative has 4 syllables and Negative has 3 syllables, so if you are expecting one or the other, if radio communications is bad, you can distinguish them by the syllables. Throwing in a non-standard A-firm, blurs it a bit for me. Generally the more syllables the word has the better it is for poor radio reception.

Both are correct. "Affirm" is not non-standard.
ICAO uses "Affirm" to mean "Yes".
However FAA uses "Affirmative" to mean "Yes".
I have not been able to find any document that addresses whether it is proper or improper for pilots to use ICAO terminology in US airspace, but US controllers use FAA terminology.
ICAO Manual of Radiotelephony: www.ealts.com/documents/ICAO%20Doc%209432%20Manual%20of%20Radiotelephony%20(4th%20ed.%202007).pdf
FAA Pilot-Controller Glossary: https://www.faa.gov/air_traffic/publications/media/pcg_basic_with_change_1_2_and_3_dtd_11-3-22.pdf


text:
Why did America not build its own ekranoplan?
The Lun-class ekranoplan (NATO name: "Duck") is a ground effect vehicle designed by Rostislav Evgenievich Alexeyev and used by the Soviet and Russian navies from 1987 until sometime in the late of 1990s. This project was huge, scattered a lot of effort, over-cost and under-delivered.

Why did the USA not design its own ekranoplan?

I'm not too familiar with your slang there, but I suspect that the answer is your "this project was huge and scatterd a lot of effort and cost a lot of bell"

Why should the USA have designed one? I'm serious; what would be its redeeming qualities, its intended mission profile(s), etc.?

@MichaelKjörling in the context of armament race, since this aircraft was military soviet project

It might be useful but not a real answer: there is (was) a concept named Boeing Pelikan though it didn't move any further from concept and I can't find any sources mentioning the project's fate and the reasons behind cancellation.

IIRC, these craft needed relatively calm seas to operate, and were therefore of limited utility.

Does the "Spruce Goose" qualify?

@MichaelHall not really, it was intended to be an actual aircraft, just using ground effect to reduce takeoff distance.

@jwenting, yeah, I was sorta kidding because it never got out of ground effect...

Best answer: the United States had no need for one.   These kinds of large wing-in-ground-effect craft, while impressive, really serve no military purpose and provide no major military advantage for the US Armed Forces to possess.
Aside from the internal Great Lakes, there are no major bodies of water within the United States of America where such a craft could be readily employed and no need to position strategic or tactical assets there.   Both the Atlantic and Pacific oceans are covered by the US Navy with the form of carrier battle groups and submarine fleets, which provide portable power projection for undersea, surface and air operations  virtually anywhere around the globe.
Large wing-in-ground-effect aircraft do not provide a significant increase in America’s  power projection when compared with that.

Indeed, the Russian ekranoplan was part of the Black Sea fleet, not an oceanic fleet.

It is rather the caspian sea fleet, since the first captions of ekranoplan were there and it was nicknamed (the caspian sea monster).

One reason the US never pursued this design is that it can't operate effectively in rough seas. It needs a relatively calm surface, which limits where and when it can be employed. A potential opponent can plan for this shortcoming.
In the early 1980's, there was a US Navy plan to conduct flight tests using the Hughes Hercules aircraft, to study ground effects. It wasn't expressly designed as an ekranoplan, but its enormous wings benefitted from ground effects (the goose never made it out of ground effects on its one flight). Sadly, that plan never came to be, and we missed the opportunity to see that giant back in the air.

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text:
Who are the audience of ADS-B?
If an aircraft has only Mode-A xpdr, then ATC with the help of Primary radar (for Position determination) and secondary radar (for receiving squawk), places a blip(only squawk code/callsign, position-from primary radar) on ATC screen (If a flight plan is there, then the controller can replace the provided squawk with his call-sign from the plan.)
If an aircraft has Mode-C xpdr, then ATC with the help of primary radar(position determination) and secondary radar (for receiving squawk and Altitude in FL), places a blip(squawk code/call sign, position-from Primary radar and altitude) on ATC screen.
if an aircraft has Mode-S xpdr, then ATC with the help of secondary radar ONLY can selectively interrogate the aircraft and aircraft sends a packet of data containing 24-bit ICAO address, position, FL, IAS, other parameters. ATC screen can display the required information on its screen (Callsign-now directly from the aircraft instead of taking from the flight plan, position-now received from aircraft, type of aircraft, FL, speed, trend-climbing or descending).
Why is ADS-B used when ATC can selectively interrogate like how Mode A/C xpdr was interrogated ?
Also, can you confirm that if an ATC screen is displaying more than callsign and altitude information, then ATC is using Mode-S xpdr only ?

An ADS-B ground station is very very simple: it consists of a stationary antenna connected to a computer which can interpret the ADS-B packets.
In contrast, a traditional (primary) radar site has a rotating antenna system utilizing GHz-range radio waves of no little power. That requires a powerful amplifier, carefully designed and constructed waveguides, a motor to drive the antenna and consumables like motor oil and bearings—not to mention the hardware required to control the radar output and interpret the input. A secondary radar site is not as complicated but still requires rotating parts.
In short, ADS-B stations are cheaper to install and maintain. That means that the same amount of money can go into more ground stations and thus provide enhanced surveillance capability in areas where old-school radar would have been prohibitively expensive.

"An ADS-B ground station is very very simple" – Scott Manley made a YouTube video about installing his own ADS-B receiver a while ago. A commenter on the video jokingly remarked that the hardest part was actually getting your hand on a Raspberry Pi.

@JörgWMittag you don't need a Pi; basically any computer that can run Linux (which is basically any computer) will work

@Someone you don't even need Linux.

A follow-up question would be... if ADS-B can do everything mode-S radar can do, was traditional secondary radar just an expensive design failure?

@fjch: Interesting question. I say no. ADS-B is dependent (hence the name) on the aircraft knowing its position in space. There have been ways for the pilot to know where they are for a long time, of course, but until GNSS receivers got small and cheap there wasn't a good way for a small aircraft like a Skyhawk to know where it was.

All you have to do is see how much of the Australian continent was Procedural airspace until ADS-B. The cost was prohibitive, but now the whole content above FL150 has surviellance coverage.
Until all aircraft are fitted with ADS-B there will be no decommissioning of mode-S radar. And that is going to be a long time away for the smaller end of town.

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text:
What is the certification process for a new propeller blade design?
I am wondering whether a propeller blade design (say from Hatzell) is submitted to a specific agency for testing and approval, and if these designs are then registered. Looking through Hatzell's product list, I would assume that there are requirements on specifications (material and form), but there must be some allowance in this?
I am also wondering if those documents are then publicly accessible, like a patent would be?

The airworthiness regulations specify functional requirements. They do not specify materials, form, etc. Thefore, the regulations in themselves do not prohibit innovation using new materials or construction techniques, so long as the final product meets the same functional safety requirements as any other design.
It is the manufacturer's responsibility to submit the required proof that a design meets the requirements. I'm not a "propeller guy" but for engines, there is usually a round of pre-certification discussion with the certifying authorities (e.g. the FAA or the equivalent in other countries) to agree exactly how the demonstration of will be carried out - for example whether by modelling and computer simulation, or by testing the finished product. In the case of computer simulation, this will include the method by which the software was verified (i.e. "how do you know that the software actually does what you think it does") and how the model was validated (i.e "how do you know if the model results are consistent with real life.")
The US certification requirements for propellers are Part 35 of the airworthiness regulations, and publicly available here.
Of course the entire process is carried out by humans, who are fallible. The regulations are often updated as a consequence of their failure - and not just for newsworthy major failures like the B787 battery fires of the B737-MAX problems.

@alphazero, thank you for your very helpful answer. Are you aware of whether or not the information regarding the submitted designs is publicly accessible?

@BtureP, intellectual property such as design information is proprietary, and generally not available to the public.


text:
Why bits are mixed up (interleaved) during ATC transponder mode A and mode C reply format?

In the image above from radartutorial.eu, we can see that the sequence of bits are c1 a1 c2 a2 c4 a4 during the reply of ATC transponder mode A.
Why are the bits not transmitted like c1 c2 c4 then a1 a2 a4. Why this mixing up or interleaving?

Mode 1 transponders only had six bits: A1, A2, A4, B1, B2 and B4, which clearly wasn’t sufficient to identify specific aircraft.  Each day, all pilots would be given a common code to squawk when returning so the air defense folks would know they were friendly, whereas a foe would only have a 1/26 chance of guessing correctly.
Mode 3/A upped this to twelve bits, adding C1, C2, C4, D1, D2 and D4. However, the C bits were cleverly interleaved with the A bits, and the D bits with the B bits, at twice the transmission rate. The result was that (for instance) Mode 1 code “12” and Mode 3/A code “1200” looked the same, at least to the primitive equipment of the era. This was specifically intended so that Mode 1 aircraft could work with Mode 3/A radar sites and vice versa, allowing a gradual upgrade. Where the new equipment was in use by everyone, individual (“discrete”) codes could be assigned to each aircraft.
This is also why, to this day, xx00 codes are mostly “non-discrete”, i.e. they have a general meaning rather than identifying a specific aircraft.

A transponder response is not directional, at least so far as I know. How did they prevent the enemy from eavesdropping on responses to know what to squawk?

@randomhead It wasn’t perfect, but it was arguably better than nothing. That’s sometimes the best you get in the middle of a war.

Probably the enemies couldn't know what to squwak because of range

@randomhead: "How did they prevent the enemy from eavesdropping on responses to know what to squawk?" Modes A/C transponders are based on IFF Mark X mode 3. Security was added in the 60s with IFF Mark XII in mode 4. Strange enough in an era of permanent cyberthreats, civilian ATC transmissions of any kinds are neither encrypted nor authenticated.


text:
Boeing 737-800 Source Off Light and Flight Director Bias Out Of View Function
I have seen the following occur during simulator events. With a left Source Off light illuminated and the captain's autopilot and autothrottles engaged (or re-engaged):

Captain's flight director lateral guidance cue will bias out of view when the autopilot is disengaged PRIOR TO glideslope capture.
Captain's flight director vertical guidance cue will bias out of view when the autopilot is disengaged AT glideslope capture.
Flight director will bias into view when either autopilot is then engaged.
Flight director will bias into view when either pilot selects TOGA.

I do not know why the above occurs. If anyone can explain this, I would be grateful.

I do not have access to any 737-800 prints. Can you please explain the "Source Off" light. Are we talking the FCC itself or primary inputs like the IRS, ADC, ILS, or any other required valids.

Interesting question. I believe the reason has to do with your cross bus tie relay (check out an image of the 737 electrical system).
When the 737 intercepts a G/S the Cross buss tie relay opens to protect the electrical system from a single source failure on an approach.
Now with that open I believe that might be affected your DC bus 1 which is connected to the CA autopilot.
Not 100% sure if that answers your questions.


text:
Why are these steps performed on the A330 emergency electrical checklist?
For an Airbus A330 in emergency electrical configuration with both generators and APU FAIL (both engines working), why does the checklist ask you to select RAT MAN ON before slat extension and then LAND REC ON?

Do you understand what each of those steps accomplishes? What in that sequence seems unintuitive to you?

Because when APU and ENG GEN are not working then you have to find an alternative source of electrical power: the RAT. The RAT is a small propeller under the belly of the plane that rotates due to the wind generated by the plane moving through the air. The RAT will deliver power to flight computers in order to send commands to control surfaces (primary and secondary)

as suggested by @RalphJ, you should explain why you think this should be ordered differently or if you think the order does not matter and thus ask why this order and not the other.

Ralph J ...The question here is WHY DO YOU PUT RATMAN  ON BEFORE SLAT EXT ? in this Senario the EDPs are working it’s only the two GEN s and APU that’s US so the EDPs  are powering the green system to power the Emergency Gen so if the Emergency gen is running why do you need the RAT ??

If we look at the Airbus A330 manual we will see that the RAT (Ram air turbine) can only power the GREEN hydraulic system and when the EDP (Engine driven pump) of the green system is available, the emergency generator is run by the EDP and not the RAT.
The A330 has two levels of emergency electrical configurations. They are:

A fault in the main electrical buses of the aircraft (AC BUS 1 and AC BUS 2). In this case, even if the engines and their generators are running the aircraft electrical system cannot be supplied. In this situation, the green hydraulic system's EDP powers the emergency electrical generator. This is the first level.

The aircraft falls into level two when there is a dual engine failure situation. So, there are no working generators to supply the aircraft electrics. Here, the RAT extends automatically. The RAT then runs the green hydraulic system which in turn runs the emergency generator.

The level one emergency configuration is always better because it provides the aircraft with more power.
When you are coming into land, while the aircraft is in level two emergency electrical configuration (RAT only), once the slats are extended, the emergency generator is automatically disconnected, and the aircraft is purely powered by the batteries. The reason for this is to allocate RAT only for the operation of the hydraulic systems so that the flight control power is prioritized. When you turn on the land recovery push button, the LAND RECOVERY AC BUS and the LAND RECOVERY DC BUS are automatically powered giving the aircraft the necessary electrical needs for the landing. This includes powering of the SFCC 1 (Slat flap control computer). But in the case of RAT powering the aircraft, only the slat channel is recovered. The landing has to be conducted flapless.

When the aircraft is in level one emergency electrical configuration (emergency generator powered by the green EDP), there is no disconnection of the emergency generator even when slats are extended. The reason why it is still required to extend the RAT in this situation is to allow the RAT to assist the green hydraulic system. The green system managing both the emergency generator and the load from the flight control demands can cause the emergency generator to shut off. With the RAT on, the green system thus gets that extra help to prevent an over demand situation. Here also, pushing the land recovery gives one of the SFCC computers. But with the EDP running the show, both the flaps and slats are available for the landing.


text:
Is the Boeing 777X being certified as a derivative of the 777 Classic?
Given the corner cutting Boeing did with the Max certification, is the FAA allowing the 777X to be certified as a derivative (easier) of the 777 Classic or requiring a new Type Certificate? (harder)
The wings are new, the engines are new, the flight deck is new (from the 787 I understand), the weight is higher and the fuselage longer.

It'll probably be certed as a 777 derivative, but the key thing is maintaining a common type rating requiring no pilot sim time to convert.  That's what drives all of this.  Can't say for certain, but in my experience the major factor in achieving a common type rating is a common cockpit with as little variation as possible between variants.  A new flight deck makes it certain in my mind that there will have to be a new type rating for crews, even if it's on the same TC.

@JohnK B737NG type ratings could be obtained in a B737C sim in the past, followed by some sessions Differences Training in a B737NG sim.

@Koyovis My type rating was in the CRJ line, and moving to different models only required a class/CBT session with no sim, even though you'd be going from a hard wing CRJ200 to a slatted, FADEC CRJ700.  When the 1000 was brought out, Transport Canada put their foot down and demanded dedicated sim time as well, although it was just a short session including demonstrating you knew how long the thing was by doing a U turn lol.  But just the requirement for a little sim was a disaster marketing-wise.

Related (general rule, not applicable to each cases, thus not a dupe): How are evolution and variants of aircraft considered by certification authorities?

Yes, Boeing applied for an amendment to the existing 777 type certificate:

On April 19, 2017 (for the Model 777-8 airplane), and May 12, 2015 (for the 777-9 airplane), Boeing applied for an amendment to Type Certificate (TC) No. T00001SE to include the new Model 777-8 and 777-9 airplanes. These airplanes are constructed with new carbon-fiber-reinforced plastic (CFRP) wings with folding wingtips.
The Model 777-9 airplane, a derivative of the Model 777-300ER airplane currently approved under TC No. T00001SE, is a stretched-fuselage, large, twin-engine airplane with seating for 408 passengers and a maximum takeoff weight of 775,000 pounds.
The Model 777-8 airplane, a shortened-body derivative of the Model 777-9 airplane, is a large, twin-engine airplane with seating for 359 passengers and a maximum takeoff weight of 775,000 pounds.

(federalregister.gov, emphasis mine)

Boeing claims it shares the 777 type rating and has a common type with the 787, which it says reduces training requirements.

Does it mean the firsts 777 and the 787 share a common type rating?

@ManuH In Europe, yes: What does “share a type rating” mean?


text:
What are the twin "air data booms" on the Su-22?
The Su-22 has two protruding "probes" at the nose. What are they? Air data probes (why two?)? Antennas?

https://www.airliners.net/photo/Libya---Air/Sukhoi-Su-22M3/1615286/L

Thats a really good question, wikipedia seems to think that these are twin pitot coupled with AoA sensor tubes.https://en.wikipedia.org/wiki/Sukhoi_Su-17#Variants The shape of the tips is definitly pitot-tube like, and the fins would fit to sensing the angles of the incoming airflow...

Just for completeness: The original-language Su-22 technical manuals are available for download here (~15$/book): http://www.aircraft-reports.com/sukhoi-su-22-aircraft-technical-description-manual-exploatation-manual-book-9-equipment-part-1-service-russian-language/


text:
What is the difference between DH and RA in Jeppesen charts?
In CAT II and CAT III Approach Charts, the Jeppesen version of charts depicts both the "DH" and the "RA".
And, I found out that they are somewhat different.
So, why do these differences occur?
Averagely about 2 ~ 3 ft deviations are existing.

Please edit and give some examples.

This entry is defined in the Jeppesen Charts Legend as follows:

The difference between radio altimeter (RA) and decision height (DH) is the different baseline. The radio altimeter will measure height above ground (directly below you) whereas the decision height uses the runway as a baseline. I made a sketch to illustrate the difference with the numbers of the example above:

It is usually not an issue for CAT III because at a typical DH of 50' you are already above the runway threshold on a normal glideslope (and for no DH it obviously doesn't apply). For CAT II with a typical DH of 100' there could be sloping terrain in front of the runway, but typically not much, that is why you see only a few feet difference. If there is a steep slope or a ditch in front of the runway, radar reflectors can be used to imitate a flat surface as shown in this related answer.
This example from Munich (EDDM) shows the RA entry only for the CAT II:

Just a reminder that RA could be considerably larger than DH, should the ground rise to meet the end of the runway. Lukla, I'm looking at you...

Thx for the detailed explanation!
I was able to totally understand what you wrote. Have a good day!

@757toga Got it, I removed all references to CAT IIIA or CAT IIIB (they were not relevant for the answer anyway). Jeppesen still prints separate IIIA and IIIB minima for other countries though (as shown in the Munich example).

@FreeMan I'm pretty sure Lukla does not have ILS and if they do, certainly not CAT II. The linked answer says that slope should be kept below 2% for 200m in front of the runway for CAT II ILS.


text:
A220 Direct Law Mode trim control
When A220's PFCCs are all failed, the airplane enters direct law mode.
In this mode, from description, pilots should manually control stabilizer trim.
When this situation occurs, does pilot need to control stabilizer trim by controlling trim switches on the sidestick?
Or is there any manual stab trim lever or wheel on board?
Plus, is there any feature that turns off speed trim bug or automatic stab trim in the simulator? (The model is CAE 7000xr)

The trim switches on the sidesticks are used during direct mode:

Sidestick pitch trim switch
The pitch trim is controlled by two pitch trim switches, one located on
each sidestick:

In normal mode, the horizontal stabilizer position is determined by the
FBW Control Laws (CLAWS). The autotrim function automatically
off-loads the stabilizer, independent of the sidestick trim switches.
In direct mode, the pitch trim switches move the horizontal stabilizer
directly.

(Bombardier CS300 / Airbus A220 FCOM 10-4: Flight Controls - Secondary Flight Controls)
There are no alternate switches on the pedestal (since the switches on the sidestick are already the alternate), nor is there a mechanical override in the cockpit, only 2 electrical motors:

Horizontal stabilizer
The horizontal stabilizer achieves the pitch trim function by varying its
angle of incidence. A dual-channel Horizontal Stabilizer Trim Actuator
(HSTA) moves the horizontal stabilizer. The HSTA includes two electrical
motors, each controlled by a Motor Control Electronics (MCE) unit. The
MCE receives commands from two REUs (AFT 1 REU and AFT 4 REU).

(Bombardier CS300 / Airbus A220 FCOM 10-4: Flight Controls - Secondary Flight Controls)
Note that depending on the type of direct mode, pitch trim speed may differ based on slat/flap position:

PFCC direct mode [...] The
sidestick pitch trim switches control the horizontal stabilizer position
directly as a function of the slat/flap position. [...]
REU direct mode [...] The pitch trim rate is a function of the slat/flap position, but is slower
than PFCC direct mode. [...]
AFCU direct mode [...] The pitch trim is like PFCC direct mode regardless of slat/flap
configuration.

(Bombardier CS300 / Airbus A220 FCOM 10-2: Flight Controls - FBW system operation - Direct Mode)

Thanks for detailed answering! Your description really helped me, have a good day!

And, could I ask you one more question?
Where can I find my current flight track information on PFD's HSI and Map Display? Only Magenta circle is my current track information?

But, even that's correct, I couldn't find any cue in FCOM that magenta circle is current track ;(

@moon AFAIK the current track is not displayed on the PFD or map. Only current heading and desired track can be seen there. You can however see the current track in the IRS information (FMS -> POS -> IRS): "TRK/GS, indicates the aircraft track and groundspeed calculated by the IRS".

Then how can I execute hand flying raw data ILS? without track information it sounds very hard in windy condition

@moon Why would you need to know current track to fly an ILS? You can set the ILS course and use the CDI (course deviation indicator) on the PFD.


text:
What is the name of the effect that causes a plane in a dive to go up?
What's the name of the effect that causes the aircraft to automatically pull out of a dive and buffet up due to aerodynamic lift produced at high speeds?

"Pitch stability" ("longitudinal stability")?  That's what's tending to hold the angle-of-attack constant at wherever you trimmed it.  "Returning to trim speed"?  "Excess lift"?  "Newton's second law of motion" (F=ma)-- a net force must cause an acceleration?  "Net centripal force = mass * velocity squared / radius"?   I'm leaning toward the last one but that's an equation not a name--!!  I think we're back to "Pitch stability" ("longitudinal stability").

PS "buffet" has a specific meaning in aviation; unless you are trying to suggest that the angle-of-attack is approaching the stall angle-of-attack, causing the pre-stall buffet to occur, you should pick another word.  "pitch up" would seem to work fine.  Maybe best to wait to actually answer till this is clarified.

An aircraft with positive pitch stability (longitudinal stability) tends to maintain the trim1 angle-of-attack, regardless of airspeed.  At any other angle-of-attack, the "teter-totter" balancing act between the wing and tail would be off-kilter, creating a pitch torque that would change the angle-of-attack.

At a given angle-of-attack, only one airspeed is compatible with level flight2.  If the airspeed is too low, lift will be less than weight (or more precisely, less than the component of weight acting parallel to the lift vector, which is relevant when the flight path is aimed steeply upwards or downwards), and this will cause the flight path to curve downwards (in the aircraft's reference frame3).  If the airspeed is too high, lift will be greater than weight (or more precisely, greater than the component of weight of acting parallel to the lift vector), and the flight path will curve upwards (in the aircraft's reference frame).

Since the angle-of-attack tends to remain constant, it follows that an upward curve of the flight path must be accompanied by a rise of the nose.

This whole dynamic is indeed one step in a "phugoid" oscillation.  Other than that, there's no one specific name for it, (except perhaps "pitch stability", or perhaps simply "pitching upwards"!)

We've made one slight oversimplification here.  In the second sentence we associated a pitch torque with a change in angle-of-attack.  In fact, any change in the aircraft's pitch rotation rate requires a pitch torque.  So some pitch torque is in fact being generated as the nose starts to rise.  This implies that the angle-of-attack cannot be remaining exactly constant.  To explain in more detail exactly what is going on here, is far beyond the scope of the original question.  (Hint--the curving nature of the "relative wind" when the flight path is not linear plays a role in altering the apparent "decalage" between the wing and the tail.)  The main thing to keep in mind is that the upward curve of the flight path is fundamentally being driven by a force imbalance, not a torque imbalance.  There's too much speed, so there's too much lift, so the flight path must curve upwards.  Meanwhile, the aircraft's pitch stability dynamics tend to maintain the trimmed angle-of-attack, keeping the nose aimed in the "right" direction in relation to the flight path, so when the flight path starts curving upwards, the nose must rise as well.

Footnotes--

The concept of "trim" is not limited to what happens when the pilot lets go of the controls or exerts no force on the controls.  It also could include what happens when the pilot holds the controls in a fixed position.

And to a first approximation, for shallow dive or climb angles, only one airspeed is compatible with linear flight, regardless of whether or not altitude is exactly constant.

Re "in the aircraft's reference frame"-- think about which way the flight path is curving in the instant during a loop when the flight path is aimed straight up or straight down.

That is part of what happens in a phugoid (the “downhill”).

A phugoid or fugoid is an aircraft motion in which the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "downhill" and "uphill".

Welcome to Av.SE!

The short period mode also causes a plane to go up and down but only for a few seconds but Phugoid(Long Period Mode) can cause the plane to pitch up and down for more than 30s.

Well, since airplanes trim to an AOA, if you force it into a dive by a down elevator input while trimmed for level flight, you're forced it to a lower AOA (higher speed) than it's trimmed for. If you let go, it will pull out because it's trying to "weathervane" (pitch stability is simply a weathervaning tendency in the vertical plane about a specific offset angle determined by trim forces) back to the AOA it is trimmed to.
So the forces causing it to pull out of the dive are trim forces, trim being an opposing force balance system of nose down and nose up pitching moments that achieve equilibrium, or trim, at some angle of the body to the airflow.
Any buffeting that occurs (like in the movies) in the real world will be due to shock waves causing flow separations and turbulence.  But for that the dive has to get into the transsonic speed range. A highspeed dive that stays within the airplane's certified speed range shouldn't experience any buffeting.
If you push it over hand hold it in a dive, while trimmed for the original speed, once you let go, it'll smoothly pull up on its own, seeking to regain its trim AOA (it'll overshoot its trim AOA while doing that and hunt up and down in ever smaller deviations until it's fully regained its trim AOA - the Phugoid Oscillation).

The "weathervaning" effect (return to trim a-o-a) would only account for a few degrees of pitch attitude change, were it not for the fact that the change in a-o-a causes a change in lift coefficient which causes a change in lift force which causes a change the direction of the flight path --

Well it's not a pure weathervane, since the weathervane is pivoting about the CG but is held up by lift forces so there is that whole interaction.  But basically the neutral point wants to trail behind the C of G, like a weathervane, and the trim force balance determines exactly what offset to the flow that that ideal trail position is found.  Yaw stability is pretty much the same, minus the supporting lift force, but if you roll 90 deg and fly knife edge like a Pitts S1, with the fuselage now the wing, yaw stability and trim forces become pitch stability and trim forces.

Is the dampened oscillation inherent? Can some aircraft and/or configurations be critically damped (or even overdamped)?

I would say airplanes tend to be critically damped or close to it in the stick fixed case, and underdamped in the stick free case (phugoid behaviour).   FAR 25 just says dynamic stability must be "heavily damped".  The test requirements do specify phugoid limits when displaced a certain amount from trim speed in the stick free case.

What is the name of the effect that causes a plane in a dive to go up?

The best answer is simply "Lift".  Lift exists, and exceeds weight (or strictly speaking, exceeds the component of the weight vector that acts parallel to the lift vector), so the flight path curves upward, and the nose rises.

It's called static stability.
A descending or diving aircraft has a gravity component pulling it forward, which increases its forward speed.
Increasing forward speed increases lift by the square of the velocity, causing the flight path to curve upwards.
As speed decreases, the process reverses, eventually settling at a velocity where flight remains linear.  We then control climbing, descending, or level flight by adding or subtracting thrust with the throttle.
One must be aware, with a tiny tail and the weight too far forward, the wing torque on the center of gravity can overpower the the tail and continue to push the nose down

this is your lawn dart

This is especially true if the wing center of lift moves backwards as velocity increases and/or downwash from the wing helps stall the tail.

"...your lawn dart" refers to William Walker III's answer (currently after in the default sort view).

@PeterMortensen but lawn darts don't have wings.  What should that tell you?

It's Called the "(Total) Aerodynamic Force"
The Aerodynamic Force is the net effect of the lift force produced by the aerofoil, and the drag force produced by the same.  Because these are integrated, the aerodynamic force tends to be net-tilted back away from the vertical.  This force acts on the center of lift (wing) rather than the center of mass of the aircraft as a whole.  As such, under certain wing conditions, this force will impart rotation in the pitch axis.

As airspeed increases, both of these forces increase, and their integrated net effect increases. At some point it's enough to overwhelm the natural resistance from the tail and becomes the dominant effect.
Where and if this happens depends entirely on aircraft design. Some craft will NOT recover like this, they'll just lawn-dart without control input (because their wings produce lift in a slightly-forward direction. Some aircraft will do this very aggressively, as a function of deliberate design (it makes them naturally recover from stalls more quickly).
AIUI, This is also why, all things equal, your nose pops up when you add flaps.

This is a nice description of the effect, but the question was about its name.

@Bianfable It's name is the Aerodynamic Force.  The rest of it is just explaining why that's it's name.

Let us continue this discussion in chat.

(Deleting comments, this answer is being discussed in chat-- )

It might help (when considering drag) to look at the entire aircraft.  Also, the "nose popping up when flaps are added" (very noticeable with a 172) is from wing down wash on the tail.  Notice a "lawn dart" would actually need a force (from somewhere in the aircraft) to drive the nose down.  You are on the right track, though, about wing area playing a role in excessive directional stability.  A plane needs to turn to pull out.

Can you add some examples of the two types of aircraft design?

That is always true for airplanes and therefore meaningless.


text:
How far does the lightning detector work at an AWOS station, as reported in a METAR?
I noticed on my local airport's METAR that there was distant lightning to the east. I am near Kansas City. When I looked at radar, the only possible system that could produce lightning was in the St. Louis area. Does the METAR really pick up distances like that (250+ miles)? What is the limit on the lightning detector? Perhaps, was there something closer that produced lightning but wouldn't have shown up on radar?

I don't know about the aviation system, but the lightning detectors used by blitzortung.org can't detect lightning that's less than about 200 miles away.

According to NOAA's Federal Meteorological Handbook No. 1, 2019 edition:

Weather phenomena beyond 10 statute miles of the point(s) of observation shall be coded as distant (DSNT) [12.7c]
Distance remarks shall be statute miles except for automated lightning remarks which are in nautical miles [12.7d]
When lightning is detected by an automated system [12.7.1j2]:

a) Within 5 nautical miles of the Airport Location Point (ALP), it will be reported as TS in the body of the report with no remark;
b) Between 5 and 10 nautical miles of the ALP, it will be reported as VCTS in the body of the report with no remark;
c) Beyond 10 but less than 30 nautical miles of the ALP, it will be reported in remarks only as LTG DSNT followed by the direction from the ALP.

This implies that LTG DSNT should not appear on the METAR if the lightning is farther than 30 nautical miles from the airport. FAA JO 7900.5 reiterates this at 13.28b.
Lightning information is collected and disseminated through the Automated Lightning Detection And Reporting System (ALDARS). This thread on a forum for Weather Observers makes for some interesting reading, for example this quote:

Talked to a friend who is NWS forecaster- they know ALDARS is shit, detects crap 200 miles away as VCTS at times- something I can also verify.

So if there was truly no convective activity within 30 miles of the airport, it may have been an issue with ALDARS.


text:
What are these white drops on the exhaust plug of a CFM56-7B engine?

Does anybody know what those white dried-up drops are?
I checked out another Boeing 737-800 on the ramp and it also had the same defect so I'm guessing this is a common issue, or is it?
This shows up the outboard side of the exhaust by the way.

Ehm - a seagull finding shelter?

Just a guess, but it looks like deicing fluid that dripped onto the hot exhaust cone just after the engine was shut down.  The origin of the drops suggests something running off the pylon side panels directly above, and the vertical tracks show that the engine was off, but the cone was still hot enough to bake off the fluid, leaving the runs of glycol residue.

Do you think deicing fluid could remain on the aircraft throughout a normal flight and then drip off after landing?  Just curious.  Sounds as good a guess as any, but-- ?

I would think it would have all burned off if it had completed a flight after that. It looks more like something dripped and then it was just parked like that.  Deicing fluid is about the only thing I can think of that would leave that sort of residue.


text:
Does the GPWS rely solely on the radio altimeter?
The ground proximity warning system (GPWS) uses the radio altimeter to receive information about the distance from the ground. Does the GPWS system work solely based on the information it receives from the radio altimeter? or are there other instruments involved with the system? If the radio altimeter gave an incorrect reading due to something like a terrain data base error would this trigger a GPWS alert in the cockpit?

Radar altimeter does not have any terrain database. It simply indicates how far to the closest obstacle straight down. Terrain database is used with GPS to also notify you of high ground (mountains) ahead where the radar altimeter is not looking.

GPWS is an older technology, and it uses the radar altimeter and a few other things, but not GPS, since it was developed long before GPS existed. EGPWS, the "E" standing for "Enhanced," uses GPS and a database of terrain & obstruction heights.  A system that has the latter will typically, in more expensive (and thus better equipped) aircraft at least,  incorporate the former, so that you have the benefit of both -- i.e. if the GPS is unavailable, you still have warnings from the radar altimeter, or if the radar altimeter fails you still have the EGPWS functionality.  That integration also handles cases when both systems would be providing an alert, so the pilots are presented with one alert rather than two at once.  However, for aircraft without a radar altimeter, EGPWS by itself is valuable, so it is possible to have the one without the other, and this is common in less expensive aircraft where the cost of a radar altimeter isn't justified.
As far as inputs to a GPWS system, it will consider things like aircraft configuration, and can provide warnings when, for instance, the aircraft is descending as if on a glideslope to a runway but without the landing gear down.  Depending on the implementation, pitot-static inputs might be considered as well.

In particular, it uses the computed position generated by the FMCs or other equivalent navigation gear -- EGPWS will still function on aircraft without GPS in their nav receivers, using INS/navaid positioning, but it's vulnerable to dodgy ground navaids in that circumstance

@UnrecognizedFallingObject In the 737 NG, that's not correct. The EGPWS has its GPS receiver, and it never sees the FMS position. I've flown when GPS jamming was active, and while the FMS position was fine (from DME/DME updates - so a slightly larger ANP), the EGPWS Terrain display was missing & we had the Fail light for the EGPWS. Other aircraft may work differently.

According to that linked article, the A-320 does pass the FMS position to the EGPWS. That incident illustrates the wisdom of keeping the position fixes separate!


text:
If spoilers are released in flight without descending, what could happen?
If spoilers are released in flight without descending, what could happen?

Welcome to Aviation.SE! Can you add some more details about the situation you are asking about? "what could happen?" is very broad...

Spoilers are also called speedbrakes. If used in level flight, the aircraft will slow down. They are usually used to slow down and descend, but they can also be used to slow down in level flight to meet a speed restriction.
On large jetliners, spoilers can also used for roll control, so it is quite common to see them deployed in all phases of flight.

Well that depends on what else you do. Spoilers will increase drag and destroy lift.
So if you are on autopilot and do not increase power(thrust), the A/P would increase AOA to increase lift sufficiently to maintain level flight, (further increasing drag), and the aircraft would start slowing down.
If you were flying manually, you would probably manually increase back pressure to increase AOA and lift to maintain level flight and increase power to maintain airspeed. This would be necessary to prevent a descent.
If you didn't take those actions the nose would drop and the aircraft would descend at an increasing speed and descent rate until your speed increased sufficiently to generate enough lift to maintain a stable descent.

In some sailplanes at least, spoilers do not cause the nose to drop and airspeed to increase.  In fact on those sailplanes they can be used in an emergency to descend through clouds with no horizon reference because the craft will enter stable ( in both roll and pitch) descending flight.

I'm not a sailplane person, but I'm curious. How does it descend without the nose dropping? And also since lift is at least initially,, decreased, won't the airspeed increase due to altitude loss to stop the glide angle from just continuously dropping?

@CharlesBretana -- well, if spoilers cause the glider to trim to a higher angle-of-attack, but also a higher sink rate-- there you go.  However if there is a substantial increase in sink rate I have a hard time believing that the nose is not going to end up at a somewhat lower pitch attitude, due to the changed direction of the flight path.  I'd want to see video evidence (or see it first-hand) before accepting idea that spoilers might not cause nose to drop at all.

Of course the whole question of how the airspeed responds depends on starting conditions.  If in an extremely steep dive (very low a-o-a), drag from spoilers may be more important than loss of lift, causing airspeed to decrease.  That's the whole point of "terminal velocity" dive brakes, which some sailplanes do have-- you can point the nose straight down (e.g. accidentally, in cloud) and not blow through redline.

@quietFlyer, yes, I guess every scenario has a spectrum of actually possible conditions under which it can occur. I'm (again), not a sailplane pilot, so I don't have any expertise on their behavior... But I agree with your skepticism. On your point about airspeed, yes, remember the Stukas in WW2! As dive angle increases, it takes less and less increased lift to maintain (or stabilize) that increased dive angle without increasing it further.

The lift required to stabilize flight at any specific dive angle is the aircraft weight times the cosine of the dive angle. And the cosine function does not start to decline significantly until about 25 - 30 degrees.

"Without descending" means that the aircraft cannot land. Therefore it will run out of fuel. In an attempt to avoid descending, it will slow and eventually stall. At this point, the conditions of the question, no descent, are violated.
As asked, the question is unanswerable.

Hah!  What if there's higher terrain ahead? (Grin)

@CharlesBretana Ohhh... interesting consideration! The homework problem didn't specify not climbing, so maybe that could be allowed.

Sorry, just injecting a bit of humor there!


text:
What is the flight purpose of this helicopter over Karlsruhe with blocked callsign?
There currently is a helicopter circling over my hometown of Karlsruhe, Germany and I wanted to find out what its purpose is.
I found it on a helicopter flight tracker but it had its callsign 'blocked'.

I do know its departure airport and I saw that it was blue and white.
Could this have been just a sightseeing flight? If so how could I find out and why was it 'blocked'?

Look it up, while it's flying, on adsbexchange. They don't block/filter as much.

@tedder42 ADSB Exchange doesn't block/filter anything. They are very explicit about this.

Whenever you see a helicopter circling over a large city in Germany, it is  most likely a police helicopter.
The helicopter took off from Baden-Airpark (Flughafen Karlsruhe/Baden-Baden FKB), where one of the police helicopters of the state of Baden-Württemberg is stationed:

Police helicopters take off from the Baden Airpark in Rheinmünster for operations throughout Baden-Württemberg
One of six police helicopters in Baden-Württemberg is stationed at the Baden Airpark in Rheinmünster. An onboard thermal imaging camera helps search for missing persons and criminals. [...]

(Badische Neueste Nachrichten, bnn.de, translated by Google)
The colour also matches your description of "blue and white".

Yeah that looks a lot like it. I was just surprised because I didn't hear anything in the radio about it

The purpose (if it was today) was most likely to survey one of the forest fires (there were several smaller forest fires in the Karlsruhe region today as well as in the last few days).

@dunni You are correct. The Police KA stated so on Twitter

Flight tracking sites make use of a technology called ADS-B to track flights. For identification, ADS-B only transmits a 24-bit ICAO address. The site has to look up the address in publicly available databases to find the aircraft that's been assigned that code.
If an aircraft shows up as "blocked", that means the site wasn't able to find the code in its database. That could be for one of several reasons.

The plane is new, or it just got a new transponder, and the database hasn't been updated yet.

There was a simple network or database error (try reloading the page).

The owner/operator of the helicopter has requested that that one specific site not show info on their plane (try other flight tracking sites, like FlightRadar24 or FlightAware).

The owner/operator of the helicopter has requested the government to not publish any information about their airplane.

If you have a set of binoculars or something, then you can try to get the registration number off of its tail. If that doesn't work, then I'm afraid your only option is to go down to the airport in question and ask around.

One website that censors even less planes than the 2 you mentioned is https://globe.adsbexchange.com/

Be aware of the privacy initiative from FAA implemented as PIA. Under PIA, the aircraft owner can receive a new anonymous ICAO address every 20 business days.


text:
Can an FAA-certified pilot rent/fly a GA aircraft in England?
I am a U.S. certificated pilot in the U.S.A. I have about 1200 hrs flying mostly general aviation.
I will be traveling to England with family in March 2019 for a personal vacation. I would like to know if anyone on StackExchange has rented an airplane in Englad before and what their experience in doing this was.
In addition, it looks like it would be pretty easy to qualify for an EASA license in England. Any advice/insight on how to go about this?

Thanks for updating the subject line..

You might get an answer from this UK forum:  https://forums.flyer.co.uk/viewforum.php?f=1

@BradWalker You're welcome. :) Good question, BTW.

You can rent a G-registration aircraft but your license will be treated as a UK private pilot ticket with day VFR privileges without any additional paperwork so long as you stay in UK airspace. There is a basic requirement to file one form, and you must get a signoff on simple theoretical knowledge, but that is it for now. See this CAA Information Notice for additional details.

I'm not challenging your answer, but a link to the CAA regulations or another source would be great.

I have updated the answer with the information you requested.


text:
Is there any evidence that the downwash vortices impact the performance of a wing?
Or is it merely flow separation on the upper surface towards the end of the wing? The assumption seems to be that the air "leaks" from under the wing to low pressure above with sufficient rapidity to overcome or offset the forward flight speed.

You've correctly called them downwash vortices. They are caused by the air behind the wing—over all of its span—moving downward, while the air outside the span does not, which creates a pair of vortices that move downward at half the speed of the downwash (and somewhat slower later as there is more air below them in their way).
The air moves down behind the wing because the lift is an upward force acting on the wing, so the corresponding reaction force is downward and acts on the air, which, being free to move, accelerates downward.
This acceleration of air, however, requires energy (because its kinetic energy increases), and because energy is conserved, work has to be done to provide it. This work is done by induced drag.
There are several other ways to show, from basic laws of physics, that induced drag is a necessary consequence of producing aerodynamic lift that is proportional to the amount of lift and inversely proportional to wing span and dynamic pressure  (that is density and square of speed). If the span-wise distribution of lift is suboptimal, it will increase the induced drag, but there is a known minimum for optimal, elliptic, distribution, and it is well known it can't be reduced below that.

The main issue with having huge wingtip vortices is vortex-induced-drag. Taking the starboard wing as an example, the air near the tip of the wing would move outboard, then upward, then inboard, then downward. So when the air hits the upper side of the wing the local flow is down and aft. This reduces the local AoA on that part of the wing, near the wingtip, thereby reducing lift and increasing drag. Also, the high pressure would have been higher and the low pressure lower if not for the vortex.
There is a second way of seeing this, from the perspective of energy. If we take the aircraft as the frame of reference, the incoming air and discharge air has the same pressure head (sufficiently far away from the wing), so if we ignore the compressibility of air all that's left is the kinetic head. The more you put into the kinetic head of the vortex, i.e. the more powerful the swirling motion of the air entrained in it, the less there is for the "normal" air just passing through and generating lift as if in an infinite-span wing. This means the bulk of the non-vortex air is slowed down more, which means more drag and less lift.

There is no non-vortex air. All of the air affected by the wing is part of the vortex.

Also the proper term is lift-induced drag. The vortices are just mechanism by which it works, but it is caused by generating lift.


text:
How do a narrow airframe and canards keep supersonic shock waves from coalescing?
In a follow up to my previous question:  How does the X-59 acheive quiet supersonic flight?
Canards and a long narrow airframe were cited as design choices to prevent shock waves from coalescing. How does this design shape achieve this goal? How does this shape affect the flow field such that the shockwaves do not coalesce?  I want to understand this in terms of the physics and aerodynamics of the shockwaves. It would be useful to see a side-by-side comparison to other supersonic aircraft like the Concorde.

After a lot of digging around, I found this.

...the main noise-reduction technique is to redesign the aircraft to
  reshape the acoustic signature. A sonic boom consists of a double bang
  on the ground that is caused by two large pressure pulses. As the
  random shockwaves travel away from the aircraft at the speed of sound,
  they pile up and coalesce. The engineers aim to reshape the acoustic
  signature of the boom by preventing sonic waves piling up.
“You shape every aspect of the airplane to make the shockwaves as
  close to equal strength as you can. When they’re as evenly distributed
  as possible along the length of the airplane, changes in temperature
  are small and the waves don’t coalesce. When the signal reaches the
  ground, it’s smeared out. Instead of two loud bangs, there’s a
  pressure change and a soft thump,” says Peter Coen, NASA’s commercial
  supersonic technology project manager.

Source: Aerospace Testing International, Dec 2018: Supersonic testing: Boom or bust?
I realize this doesn't answer the question of the physics, but I thought i would stick within my paygrade. Someone else who better understands what the fluid is doing can tackle explaining that.


text:
Why doesn't the centre of pressure move in the case of a symmetric aerofoil?
Why doesn't the CP (centre of pressure) move in the case of a symmetric aerofoil?
Also, why does the CP move backward in the case of a flat plate?

Have you checked out Peter's answer to https://aviation.stackexchange.com/questions/30615/does-the-centre-of-pressure-of-a-cambered-airfoil-go-ahead-of-the-quarter-chord? It seems to go after your same question.

The cp on a flat plate only moves back when separation has started. The same will happen on an airfoil (symmetric or not).

Simple answer:
The pressure difference between lower and upper side on an uncambered airfoil or a flat plate can be summed up in a force acting at the quarter point, independent of angle of attack. Only when separation starts will this pressure distribution change shape and the force will move backwards. Since a flat plate has a sharp leading edge, separation happens at much lower angles than on an airfoil. If you think this is tautological, please read on.
More precise answer:
In potential flow theory, circulation strength is proportional to the local pressure difference on both sides of a body. When I now talk about circulation, just think pressure difference.
The distribution of the circulation strength $\gamma$ over an uncambered airfoil is given by Birnbaum¹ as $$\gamma = \sqrt{\frac{1-x}{1+x}}$$ for the lengthwise coordinate $x$ running from -1 at the leading edge to +1 at the trailing edge of the airfoil. Like this:

Now it is important to note that the shape of this distribution stays the same, regardless of angle of attack. Only the intensity of the circulation varies with angle of attack. Until separation starts, that is.
If you calculate the centroid (= center of gravity of the area under this curve), it lies at -0.5. If we now translate the X coordinate to the more usual 0 at the leading and 1 at the trailing edge, this center of gravity sits at 0.25 or the quarter chord point. And it stays there, regardless of angle of attack (again, until separation starts!). Circulation is equivalent to lift, so the same holds true for the pressure coefficient over chord.
This explains why the pressure distribution of a symmetric airfoil stays at the quarter point. It does so, too, in case of the flat plate, only that the angle of attack range where this is valid is much narrower. The sharp corner at the leading edge of the flat plate will cause air to separate already at small angles of attack, so the validity of potential flow theory (of which this proof is part of) is limited in case of a flat plate. Once separation starts, the pressure over chord tends to stay constant, so the center of gravity of the pressure distribution shifts towards X = 0.5.
¹ W. Birnbaum: Die tragende Wirbelfläche als Hilfsmittel zur Behandlung des ebenen Problems der Tragflügeltheorie. Zeitschrift für angewandte Mathematik und Mechanik 3, pages 290-297 (1923)
https://doi.org/10.1002/zamm.19230030408

Omg I understood nothing, but better give +1 just in case...

@Jpe61: It's hard to find the adequate explanation when I don't know where you stand. If you let me know what's unclear, I can start over and write the same thing with less math.

Perhaps it would aid understanding if you refer to the centroid of the area under the curve instead of the center of gravity, which may be confused with a plane's center of gravity? Not sure how else one would improve this clear and concise answer.

@PeterKämpf if you could simplify the explaination pls !!!

@skyhigh The simpler answer is close to circular reasoning, but here you go.


text:
The latest turbofan aircraft feature an exposed exhaust cone, does it come with some benefits?
Aircraft like Airbus A320neo/A330neo/A350 series have a visible exhaust cone, which is not visible in the older versions of Boeing 747, Airbus A330/A321/A319 etc. Why is this design preferred over the other?
Image: Exhaust Cone not exposed
Image: Exhaust Cone exposed

Note that on A320CEO the CFM56 engines have outer and inner nozzle and the exhaust cone just barely visible, but the IAE V2500 engines have common nozzle and the exhaust cone is fully inside.

An image of 'visible' and 'not visible'  would be very helpful.

Welcome to aviation.SE! Does this question help?

@skipper44 I've updated the question with Image URL, Thanks for pointing out.

@Pondlife Thank you! I can't say exactly how, mainly because I'm not an experienced aviation person. I've updated the question with the image URL and when I compare those images, it seems that the engine with exposed/visible exhaust cone may have better bypass ratio maybe?

The « cone exposition » is a result from nacelle engine design, and the choice to mix flows or not. The two main strategies are called:

Mixed exhaust engine resulting to long nacelle hiding exhaust nozzle and plug.

Separate exhaust engine which is shorter and then allows a view of nozzle component.

The reason to choose one type of exhaust is made by considering overall engine performance (mainly efficiency and fuel consumption), noise but also visual pleasing (for business jet for instance).
Exhaust mixer is generally more efficient (secondary flow is slightly accelerate producing more thrust) and less noisy (primary flow is slow down). They might be also an advantage in material resistance in cooling hot primary flow. See this article for details about advantage of mixed exhaust.
Nevertheless, long nacelle has the overcome to increase weight and drag of the engine and this is worst as the engine is bigger. Note that it is related to turbofan by-pass ratio (BPR). More the BPR is high and bigger the engine diameter is. For commercial aircraft, the trends is to increase more and more the BPR parameter because it is directly correlated to propulsive efficiency. Therefore, there is a kind of « BPR limits » where mixed nozzle benefits do not balance its drawback.
That's why, modern turbofan (high BPR and high diameter) have likely to integrate a separate exhaust nacelle.

It depends on the airframe/engine combination rather than than newer and older airplanes.
Very generally, P&W have mostly had visible exhaust cones when compared to GE and RR, atleast on widebodies.
See this image of the P&W JT9D on the wing of a 747-200., the RR RB 211 on the wing of a 747-400), and the GE CF6 on the  wing of a 747-400


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What is the significance of radar cross section (RCS) for air defense?
From the internet or any other medium of information I came to know that some cutting edge stealth aircraft have radar cross sections of a bird (or something similarly small).
I don't have the in depth technical know how of how modern radars, air surveillance and detection systems work (obviously). But judging from what they are required to be, here is my conclusive query:
If you want to detect an enemy aircraft (either using a centralized monitoring system or in an anti aircraft missile), you want the detection in the 3D cartesian (x, y, z) axis location of it. If that is possible, and an aircraft with a bird size of RCS, is seen as a bird or small object on radar, still a small object at heights of 30,000 - 40,000 feet moving at a speed of near Mach 1 or higher Mach numbers, would not that be something very obviously alarming?

Gotta love Heisenberg's Uncertainty Principal. Radars are a perfect example of this coming into play. Radars cant perfectly see both location and speed. The more accurate they are on one, the less accurate they are on the other. A smaller RCS dramatically complicates this.

@DavidS  Err, what? How do you work that out? A slow sweep-rate will affect both; a faster sweep will surely make both readings more precise?

@MikeBrockington Its a balancing act among multiple things. Too fast of sweep and you lose accuracy on speed readings. Think of a single pulse. One ping only. Great for distance and location. But that second ping is needed to update location, or obtain speed.

@DavidS I think, modern systems must be having redundant but optimized systems to compensate for these difficulties.

@DavidS - Why don't you think radars can't "perfectly see both location and speed"? Modern day pulse-doppler radars can do both, and on a single pulse if the SNR is good enough.

@SteveSh A single radar wavelength pulse has a lot of difficulty in obtaining speed from a target, as you admit with SNR being good enough. Modern systems have redundancies, and methods to compensate for this. I don't dispute a modern radar system to see a target's speed and location simultaneously. I failed to appropriately distinguish between the radar system and radar signal. However, the uncertainty principal is very much a factor in radar technology. Got a paper as recently as 2019 attempting to address it: https://arxiv.org/pdf/1901.07994.pdf

Who said it was a single wavelength pulse?  At 8 GHz, a 10 microsecond pulse has 80,000 RF cycles in the pulse.

Its all about detection distance.
Take a car and a beer can. You're 10 miles away, on an open plain. From there, you can't see either. Get closer...5 miles. Still can't see either.
At 3 miles, you can see the car, but not the beer can. So there, you can shoot or avoid the car.
You can't see the beer can until you are 100 meters away. But the beer can is actually the same physical size of the car, and just as deadly.
A smaller radar signature gives me time to get closer, without being detected.
To bring this out to the real world, B-52 and B-2. The radar signature of the B-52 is B-52 size. The B-2, bird size. Much much smaller.
So...
You have radar sites all around your border. A B-52 is flying toward you. You can detect him 500 miles out.
Scramble your interceptors. They find him 200 miles out, and either convince him to go away or shoot him down.
The B-2, however...you might not detect him until he is only 5 miles out.
By the time you scramble your interceptors and they find him, he is already inside your perimeter, doing unspeakable things to your countryside.
Detection distance.

"But the beer can is actually the same physical size of the car..." -- I'm not sure what you mean here.  My beer cans are much smaller than my car.  Can you elaborate?

@WayneConrad: A bear maybe. A whole bear in a big can.

@Wayne Conrad - That is why I edited in the B-52/B-2 comparison. Mostly the same physical size, vastly different radar signatures.

@WayneConrad how about: "but - SURPRISE - the beer can turns out to be the same size as the car, despite how it looks"

A bit on the beer can/car analogy.  This holds assuming both have the same reflectivity.  But what if they don't?  Assume they're in the desert.  If the car is painted desert camouflage, while the beer can has a mirror-like finish, then you may be able to detect the beer can at a greater distance than the car.  This is what RCS is all about - minimizing the returns (reflections) back to the radar site.  And the size of the object is only one part of that.

@SteveSh you can take the analogy further - your shiny can helps only if it catches the sunlight, or if facing directly towards you and you shine a light on it in the dark (that last being basically the same as radar of course)

@SteveSh that kind of thing is what is being measured by "cross-section". If the car is as easy to see as a beer can then it has "the cross-section of a beer can". Doesn't matter whether you achieved that with special paint, special geometry, or a slightly torn invisibility cloak

My issue is with "the same physical size", which a beer can is not.  In every dimension a beer can is an order of magnitude different in size than a car.  I can't get past this sentence that seems plainly incorrect to me.  Did you mean some other adjective than "physically?"

@WayneConrad - Instead of car+beer can, consider 2x pickup trucks, with a .50 CAL mounted in the bed. We sprinkle some magic dust on one of them, known as Stealth Paint, and it then appears to the naked eye as the size of a beer can. The gun is still just as deadly. But you are in range of its gun, long before you can see it.

@WPNSGuy That's a pretty good analogy

Yes, I'm a confused about the car and beer can analogy, but the crux of the answer as I get it is that:
RCS alone is useless info. What is crucial knowledge is RCS at a particular distance from RADAR under standard and worst case atmospheric conditions and of course that has to be super classified data.

Besides the problem of whether you can see it at all far enough away to have time to do something about it, you also have the problem that if you do build a radar capable of spotting bird-sized objects at 100 miles, then you get to see every bird-sized object within 100 miles of you on your radar screen. And there are lots of birds in the world!
Every time the radar sweeps round you get zillions of dots, and then it sweeps round again a few seconds later, and you get zillions of dots again in different places, and you have to figure out which pairs of dots are from the same object, moving in some unknown direction, and which are different. Over and over again, with every sweep. Join-the-dots is fairly easy if you only have a dozen big chunky dots on the screen and they're well spaced out. The more sensitive you make the radar, the more dots there are, the more closely spaced they are, and the harder it is to tell if they're the same thing. An aircraft moving at 330 m/s might be half a mile away by the time the radar beam sweeps round again. How many birds, reflections off moving ocean waves, or waving tree branches, glint off buildings, mountains, cliffs, etc., can you see within half a mile around each and every dot?
The more sensitive you are forced to make your radar, the harder it is to process all the zillions of tiny objects you can now see, and make sure none of them is an approaching threat. And the more reliable you have to make your processing. If you have a hundred radar tracks, and 0.01% error rate per sweep on each track (99.99% correct sounds pretty good, right?), you would get about 1 false alarm every couple of minutes. If you have ten thousand radar tracks, you would get 100 false alarms every couple of minutes!
Making the radar sensitive enough that it can see birds at very long range makes the radar's job much, much harder.

Well, this only applies when Doppler shift of the reflected signal is in the range you would get from the bird, which is sometimes, but not always. Same for the altitude, I think.

@EugeneRyabtsev but Doppler radar has its own downsides - you need to dwell on an area of sky for longer and getting the frequency of every reflected signal still isn't easy when they're close together.  The Doppler effect can be small, even zero if the target is moving tangentially

"Making the radar sensitive enough that it can see birds at very long range makes the radar's job much, much harder."  No, that's not entirely true.  There are signal processing and filtering techniques that can be used to remove the returns from thousands of birds, without dialing down the sensitivity of the radar system.  I can't think of any military radars that deliberately compromise sensitivity, except in very specific situations.

@SteveSh  signal processing is great, but this answer already points out that it is harder than not having to do it at all.

@MikeBrockington This gives a rather ancient notion of how a military radar works. If it was just about connecting the dots, ECM would be so much more effective.

@EugeneRyabtsev: Modern radars are indeed more complex, but their problem with ECM remains. It's easy to create a thousand false signals with the same amount of energy as your stealth bomber's reflection. You can't assume the false signals are all natural and conveniently distinguishable; some of the false signals are intentional

Mike Brockington - In today's world, that sounds like a rather contrived, artificial, and not very relevant viewpoint.  Unless you're building a simple hobbyist radar with soup cans for antennas.

@MSalters Combining the input of different radars with different operating frequency plus filtering on track speed and persistence and passive radar signature (all done automatically) it is not so difficult to remove false signals.

A radar's antenna sends out a radio signal that is reflected off of the target.  That reflection, the "radar return," is received by the radar's antenna. The radar has a certain minimum amount of energy it must receive before it can register the return.  Returns that carry too little energy won't be registered.  They won't show up on the scope.
That means that there are a couple of ways to keep your airplane from showing up on the radar.  One is that you can be too far away.  As an airplane gets farther away, less radar energy gets to it to be reflected back to the radar.
And, the less of that return is received by the radar's antenna.  The radar signal is attenuated by the inverse square law twice, so the energy of the radar return is inversely proportional to the 4th power of the distance.  If my airplane gets far enough away, the radar doesn't see it.
Another way to keep the airplane from showing up on radar is by having a lower radar cross section.  The lower the radar cross section, the less the radar signal is reflected by the plane.
If I can make my plane's radar cross section be very small, then I can get much closer to the radar before I am seen than if my radar cross section was large.

Even if two objects, say a bird and a plane, have the same RCS and so can be detected at similar ranges, there are ways to distinguish between the two of them by the way they interact with or reflect the radar signal back to the radar receiver.
Modern day radars can not only distinguish between a bird and a plane, but in many cases can distinguish between types of planes.

Fortunately, they can distinguish between a bird and a plane, but can they identify Superman?

Yes!  By the flapping of his cape ;-).

One more reason why capes are a serious safety issue. No capes!

I would hope that modern day radars have the option to filter out a Superman return, because other than the novelty of seeing Superman, there isn't anything you can do about it. Unless you have Kryptonite.

Your question is valid and you are right to suspect that in many situations having a reduced RCS is not useful. Speaking as someone with experience in military radar research I can describe some of the basic factors:
(1) Having a reduced RCS simply increases the chance an aircraft might go undetected and does not eliminate the possibility of detection.
(2) Reducing RCS is very effective against intercepting aircraft because such aircraft have weak radars that operate head on. This makes it difficult for interceptors to find your bomber or to target it with air-to-air missiles.
(3) Reducing RCS is effective against weak ground based radars and reduces the range at which they can detect the bomber. This makes it a lot easier for a bomber to penetrate the borders of a country without being noticed.
(4) If the bomber is operating in a limited area that has air defenses or in an area where the enemy expects the bomber to pass, then the reduced RCS is more or less useless. We saw the effects of this in the 1999 F-117 shootdown.

That is an interesting incident and an informative article! But, how can it be concluded that RCS is useless here? The wiki doesn't share any info on how precisely were they able to pin point the plane and take it out? Are there any similar incidences involving modern 4th or 5th gen stealth aircrafts?

@lousycoder In that situation the lowered RCS was useless because the Serbs knew the path the bomber would take and they also knew approximately when it would arrive because they had spies in Aviano that watched it take off. So, when the aircraft arrived in the designated defense zone they just turned on their radar and shot it down. The fact that it had a lowered RCS didn't matter because once it is close enough to the radar it can still be detected.


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Is it a good idea to increase exhaust duct length?
I am a big newbie in aviation but still, I have a question:
Let's assume a civil aircraft (Boeing 737 or Airbus 320), flying with those impossible real life hypothesis:

the exact SAME turbofan are used
the turbofan has a MIXED exhaust engine (so cold and hot air are mixed before ejected)
aircraft is flying at the SAME altitude
altitude temperature is the SAME
pressure is the SAME

Now, let's consider the following scenarios:

Scenario A: classic design
Scenario B: classic design but entry length is increased
Scenario C: classic design but exit length is increased
Scenario D: classic design but entry and exit length are increased

About the added pipe:

same material
same diameter for the whole beginning part
same diameter for the whole ending part

Notice that for the purpose, we do NOT care about:

if the added pipes can touch the runway then landing or taking off
the equilibrium point of the motor/aircraft
the weight it could add to the aircraft (thus consumption, flying distance, etc.)
the feasibility to implement it (structurally possible or not)
the stealth of the aircraft (I mean, about thermal imagery and whatever)
the external design shape of the turbofan shell

My questions:
Would the motor be more efficient at generating thrust?
My thoughts

Should limit/decrease noise reduction (unidirectional)
Should limit/avoid turbulence around the first blades ring
Should redirect more reaction power to a specific direction a thus provide better efficiency

Closed?  Wow.  Intakes can be longer and even S shaped.  Longer exhaust would just add weight.

I don't really why why they closed it :/ Anyway, indeed, it will add weight but do we gain in term of noise, performance, protection, turbulence, etc ? That's my real question :)

The 'needs focus' close is coming from the question modifying the intake, the exhaust and also mixing in high bypass turbo fans and classical turbo jet. Suggest removing the intake section and if need be splitting it off into a new question since intake flow is a complex question in and of itself, with unrelated physics. It would also help to name check and use illustrations for pure turbine engine, with no bypass fan since that simplifies the physics.

@user12642493 I have submitted an edit to better match english language usage around jet engines, but would still be good if you did an edit to make it just a question about exhausts OR intake duct length. In particular 'reactor' would mean the actual combustion area of the engine, which does benefit by being made longer to allow more complete reactions to take place but would be a different question again.

I accepted the edit, what happened?

This really will make a difference if it's a mixed exhaust or separate exhaust engine

I've voted to reopen the question (@MichaelHall the original question was much more convoluted and asked several questions).

Thanks all for your explanations, now I better understood about the "closed" question. I wasn't aware that mined/non mixed exhaust would make such a difference. I will then update my question accordingly

The longer the exhaust "pipe" is the more weight and instability you add to your aircraft and the longer it is the more likely it is to hit the runway during takeoffs and landings. It also more effective to push the exhaust in a wide area behind the engine instead of a small area.

Would the motor be more efficient at generating thrust?

No. Both the inlet's and the exhaust's lengths are already optimised for cruise conditions, which is where the engine is going to spend most of its life.
Regarding the inlet, its main job is to reduce the speed of the incoming air to some Mach 0.4, whatever high the speed of flight is: Mach 0 to 0.4 is the limit within which the compressor's blades (and especially their tips) work in a proper way.
If the speed of flight is higher than Mach 0.4 then the inlet simply works as a divergent inlet (diffuser): its section gets bigger and bigger, the pressure gets higher and higher (so called pressure recovery) and the speed gets lower and lower (the compressor thanks Bernoulli). A shorter inlet would waste some of the pressure recovery while a longer one would simply weight more and generate pressure losses. The same reasoning applies to the exhaust outlet.


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What would be the Sref i.e. Reference Wing Area for Drag Build Up Calculation in case of a box wing design?
Would Sref be sum of areas of both, the front and the back wing? or just one wing? Assuming both the wings have equal areas (image below)
Details - So, I am calculating drag using the component drag build up method and using the below equation for the following parts of my box wing design.

Front Wing
Aft Wing
Fuselage
Tail

as you can see in the equation below, Swet for all different parts are divided by "Sref". For example assuming that both wings have area of 10m^2 and therefore wetted area of roughly about 20m^2 each.
for Cd0 calculation what should use as my Sref? total lift generating area of the aircraft, as in 10+10=20? or just the area of any one wing, 10?
If I use 20, for example, and apply the below formula for the front wing, my (Swet/Sref = 1) whereas it's = 2 if I use, Sref as 10m^2?
Basically, my total CD0 values changes drastically as I change my Sref according to the below equation, why is that?

You could argue for one or for both, but ultimately, it doesn't matter - as long as you are consistent and use the same reference area throughout your calculation/analysis.
If I personally were to choose, I would go for both, on the basis of total lifting area.
EDIT for details ------
Well yet again, it doesn't matter. You will get some value for your $C_{Dmin}$. Then later for other calculations you will use the same $S_{ref}$ and you will get comparable drag coefficients. Finally, when you use $D_{total} \text{ [N]} = C_{D} \cdot 0.5 \cdot \rho \cdot v^2 \cdot S_{ref}$ you will input same $S_{ref}$ and you will get the same Newtons regardless of chosen $S_{ref}$.
What matters is that you're consistent, i.e. if you use $S_{ref} = 10 \text{ m}^2$, use that for everything. If you use $S_{ref} = 20 \text{ m}^2$, use that for everything.
Additionally, do recheck the wetted area argument. $S_{wet} \neq S_{ref}$, because $S_{wet}$ includes both upper and lower surface of each wing, whereas $S_{ref}$ includes only the planform surface.

Hi Thanks, yes I agree! I have been using that assuming Sref means the total lift generating area! However, my drag significantly decreases, which looks too good to be true, and if I halve that Sref, total drag increases.

@Leo Your total drag in N should be independent of the reference value you choose. Can you elaborate more how you calculate the drag, and how the value of Sref is used in it?

Something's not right there. $S_{ref}$ serves to calculate lift and drag coefficients, so it should not affect your lift and drag forces [N].
Maybe you could add more details so we would be able to point you better.

Hi, thank you, I have added the details above